Adaptive control method for refrigeration systems

ABSTRACT

ADAPTIVE CONTROL PROCEDURE FOR REFRIGERATION SYSTEMS which comprises the detection of the frost level in the evaporator by means of a calculation method of NTU rate, allowing to define: the most suitable defrosting time, the energization of the drainage resistors and the adaptive management of the evaporator fan combining different operating modes. An ice-free mode which uses only the cooling capacity of the refrigerant, and different modes with ice which benefits from the latent heat stored in the ice to produce energy savings, depending on the level of frost in the evaporator. For calculating the NTU rate, it uses the evaporator as a reference when it is dry at the beginning, and when the cooling system is in operation, it calculates the NTU rate with a variable frequency operating mode depending on the evaporator performance or level of ice and its comparison with the cited reference.

OBJECT OF THE INVENTION

The invention, as expressed in the title of the present specification, relates to an adaptive control method for refrigeration systems, providing advantages and characteristics, to be described in detail below, entailing an improvement in the current state of the art within the field of application thereof.

More specifically, the object of the invention focuses on a control method for refrigeration systems, this being adaptive on the basis of the level of ice in the evaporator; for this purpose it monitors the refrigeration system and manages the fans and the defrosting processes in accordance with the level of frost in the evaporator, this entailing significant energy savings in the refrigeration system. Furthermore, the level of frost in the evaporator is detected by means of a new calculation method which is valid for any type of system and which is based on an NTU (Number of Transfer Units) rate method.

SCOPE OF THE INVENTION

The scope of the present invention is included in the industrial sector devoted to the manufacture of refrigeration equipment, focusing more specifically on the operation control systems of the same.

BACKGROUND OF THE INVENTION

It is well known that the efficiency of refrigeration systems can be reduced due to the formation of ice (frost) in the heat exchanger (evaporator) circuit of the refrigerated space (evaporator). If this excess frost is not prevented, it may even halt the evaporator [1]. There exist several defrosting methods; some require large amounts of power to eliminate said frost [2] as much as 25% of the total power consumption of the refrigeration system [3]. It is known in the sector that a reduction in the frequency of defrosting may improve the performance of the refrigeration system, as its power consumption is reduced.

It is for this reason that in general, defrosting processes should be maintained at a minimum.

Generally, defrosting processes are programmed at particular times, typically every 6 or 8 hours, with no information regarding the state of the evaporator, which causes on the one hand possible unnecessary defrosting processes, and on the other, periods where there is excessive frost.

The evaporator fan may be managed in different ways, depending on the level of frost in the evaporator, in order to reduce the power consumption of the refrigeration system [5].

In view of the above, the object of the present invention is to develop an improved control method for refrigeration systems, based firstly on a new method for the detection of the level of frost in the evaporator, and secondly on the adaptive management of the evaporator fan so that it may combine different operating modes, and finally, an adaptive criterion to establish the most appropriate defrosting time.

It should be mentioned that said new method for the detection of the level of frost is based on the well-known NTU (Number of Transfer Units) method, used to calculate the heat transfer rate in heat exchangers (particularly upstream heat exchangers) when there is not sufficient information to calculate the logarithmic mean temperature difference (LMTD). In the analysis of the heat exchanger, if the inlet and outlet temperatures of the fluid are specified or can be determined by the simple energy balance, said LMTD method may be used; however, when these temperatures are not available, the NTU method is used.

Additionally, and as a reference to the current state of the art, it should be noted that, although operation control systems for the fans in refrigeration equipment for the optimisation of their operation are known, at least by the applicant, the existence of a method presenting similar or identical characteristics to those advocated herein, as claimed, is unknown.

In this sense, the existence is known of documents EP0328152 of 1992 and U.S. Pat. No. 4,949,548 of 1990, [5, 6] referring to a patent relating to the control of evaporator fans in such a way that the cooling capacity stored in the ice in the evaporator is used, by melting the same and ensuring that the cold is effectively transferred to the refrigerated space; however, the method employed presents significant differences. Specifically, in said document a control based on the temperature difference between evaporator and refrigerated space is used to quantify the level of frost in the evaporator and thus to programme (decide) the commencement of the defrosting process. This approach to the problem is valid, although its application is limited solely to self-contained refrigeration systems (that is, with a condenser unit devoted to the evaporator in question).

Likewise, in patent application US2005/0132730 [7] the method ε—NTU is proposed for the management of the fan of a commercial refrigerator.

In the present invention, to be described in the sections below and whose scope of protection is defined in the attached claims, the quantification method (NTU-rate) is different from those proposed in [5, 6, 7], and specifically enables said control to be valid for both self-contained systems and for those featuring centralised condenser units formed by racks of multiple compressors; this representing a significant advantage.

REFERENCES

-   1. Claudio Melo Fernando T. Knabben, Paula V. Pereira. An     experimental study on defrost heaters applied to frost-free     household refrigerators. Applied Thermal Engineering 51 (2013)     239-245. -   2. J. M. W. Lawrence, J. A. Evans. Refrigerant flow instability as a     means to predict the need for defrosting the evaporator in a retail     display freezer cabinet. International Journal of Refrigeration     31 (2008) 107-112. -   3. Kazachi G. Project progress meeting in discussion of display case     warm liquid defrosting test at EPA. Raleigh: 2001. -   4. Diogo L. da Silva, Christian J. L. Hermes, Claudio Melo.     Experimental study of frost accumulation on fan-supplied tube-fin     evaporators. Applied Thermal Engineering 31 (2011) 1013-1020. -   5. Friedhelm Meyer, (1990). Process for controlling the operation of     a refrigerating unit. U.S. Pat. No. 4,949,548. -   6. Friedhelm Meyer, (1992). Verfahren zum Steuem des Betriebs eines     Kühlaggregats. EP0328152B1. -   7. Lim Hyoung K et al. Apparatus and method for controlling     operation of blower fan of refrigerator. US2005/0132730.

DESCRIPTION OF THE INVENTION

The adaptive control method for refrigeration systems proposed by the invention is therefore configured as an innovation within its scope of application, the characterising details distinguishing the same being appropriately included in the final claims accompanying the present description.

As has been mentioned above, the invention proposes an adaptive control method for refrigeration systems based on the level of ice in the evaporator, which monitors the refrigeration system and manages the fans and the defrosting processes in accordance with the level of frost in the evaporator, conferring significant energy savings on the refrigeration system, comprising essentially a new method for the detection of the level of frost in the evaporator, the adaptive management of the evaporator fan which intelligently combines different operating modes, and finally, an adaptive criterion to decide on the most appropriate time for defrosting.

Specifically, the level of frost in the evaporator is detected by means of a new method for calculating the NTU rate which, advantageously, is valid for any type of system.

The control method therefore combines different management modes of the evaporator fan in accordance with the level of frost in the evaporator, which is in turn determined by said NTU rate method, causing the refrigeration system to operate in different operating modes:

-   -   Ice-free mode: Solely the refrigeration capacity of the coolant         is employed.     -   Measurement mode: This mode enables a precise NTU rate         measurement.     -   Different iced modes: The iced modes employ the latent heat         stored in the ice to provide energy savings, depending on the         level of frost in the evaporator.

The adaptive control method of the present invention, according to a first aspect, comprises the performance of the aforementioned detection of the level of frost by means of the obtaining of a dimensionless coefficient fc of the relative level of frost in the evaporator and the monitoring of the temporal evolution of the same, where the method comprises the obtaining of said dimensionless coefficient fc of the relative level of frost in the evaporator:

-   -   from the calculation of a first value or reference value of the         NTU rate, performed when the evaporator is dry at the         commencement, with no frost, and     -   from the calculation of second values of the NTU rate, when the         refrigeration system is in operation during one of said iced         modes of fan management, performing said calculation repeatedly         over time, with an inconstant frequency of repetitions which         varies depending on the performance of the evaporator or on the         level of ice in the same;

where said dimensionless coefficient fc of the relative level of frost in the evaporator relates, in a comparative manner, the second values with the first value of the NTU rate.

In other words, the adaptive control method of the invention, in accordance with said first aspect, contemplates the calculation of the NTU rate at the commencement, when the evaporator is dry (with no frost). Said level is used as a reference. When the refrigeration system is in operation, the adaptive control method contemplates the repeated calculation of the NTU rate, with a variable frequency of repetitions (depending in turn on the output of the evaporator or the level of ice therein), and their comparison with the reference. The value obtained is a dimensionless coefficient (fc) reporting on the level of frost in the evaporator.

Depending on the fc coefficient, the strategy (mode) of operation of the evaporator fan is decided, and it is decided whether a defrosting process is required in real time.

To this end, in accordance with an embodiment, the fc coefficient is compared with the value of a dimensionless reference performance coefficient fs indicating that a defrost is required, which in turn adapts, subsequent to said comparison of fc and fs values, being updated in accordance with the time required to perform the defrost on implementing one of said iced operation modes on the basis of said value of fc compared, the first fs being a default value. Thus, the value of defrost activation is adapted until a level of frost is achieved in the evaporator which enables the obtaining of the optimal (most efficient) level of operation of the refrigeration system.

A more detailed explanation is provided below of an embodiment of the first aspect of the present invention, concerning the new method of calculating the NTU rate employed to detect the level of frost in the evaporator; a method which, advantageously, is valid for any type of system.

Specifically, the calculation performed in accordance with said embodiment consists of the relative assessment of the heat flow lost by the air in the refrigerated chamber at the moment when coolant enters the evaporator. According to the classic ε-NTU method, a mode of quantifying the heat flow lost by the air in the chamber obeys equation 1:

q=ε·Cp(air)·{dot over (m)}(air)·(T _(air) −T _(evap))  (1)

Where q is the heat flow absorbed by the evaporator, ε is the efficiency of the heat exchanger, Cp(air) is the specific heat of the air, {dot over (m)}(air) is the mass flow of air crossing the fins of the evaporator (driven by the evaporator fan) and (T_(air)−T_(evap)) is the temperature difference between the air in the refrigerator chamber and the evaporator, which is assumed to be constant throughout the evaporator (as the coolant is evaporating).

The heat flow “stolen” by the evaporator from the air in the refrigerator chamber is constant, as:

-   -   the air in the refrigerator chamber is at a controlled         temperature and therefore has a constant [Cp(air)].     -   The air flowrate responds to the evaporator fan, which has a         constant flowrate [m(air)].     -   The flowrate and the enthalpy leap of the coolant in the         evaporator are adjusted by means of the control and power of the         compression (constant) and expansion; they are therefore         constant.     -   The temperature of the evaporator, where the coolant changes         phase, is constant throughout the entire evaporator.

Therefore, when there is no frost in the evaporator, its exchange of heat with the air in the chamber responds to a characteristic performance (ε dry). Conversely, when the evaporator has a particular level of frost, the exchange of heat responds to a different performance (ε ice). The performance drops simply because the frost represents a thermal insulant for the exchange of heat. However, in both cases, the exchange of heat is the same.

Therefore:

q _(dry)=ε_(dry) ·Cp(air)·{dot over (m)}(air)·(T _(air) −T _(evap))^(dry)  (2)

q _(ice)=ε_(ice) ·Cp(air)·{dot over (m)}(air)·(T _(air) −T _(evap))^(ice)  (3)

If the equations are equalised (due to the fact that, as has been mentioned above, the heat flow is constant with and without frost) it may be observed that:

ε_(dry)·(T _(air) −T _(evap))^(dry)=ε_(ice)·(T _(air) −T _(evap))^(ice)  (4)

From the analysis of this equation it may be observed that the gradient between air in the chamber and evaporator changes linearly with the relationship between efficiencies of the heat exchanger.

By means of the precise measurement of both temperatures (T_(air) and T_(evap)) under frostless (dry) and frosted (ice) conditions, the loss of performance of the evaporator may be determined.

For this reason, to implement the method of the present invention, a system featuring two temperature probes is used:

-   -   Chamber probe: measures the temperature of the air in the         chamber (and thus regulates the cooling necessary to maintain         the chamber at the desired temperature).     -   Evaporator probe: measures the evaporation temperature of the         evaporator, in contact with the piping where the coolant expands         and evaporates.

Knowing that the exchange efficiency is related to the NTU for evaporators according to:

ε=1−e ^(−NTU)  (5)

Where NTU is

$\begin{matrix} {{NTU} = \frac{UA}{\left( {\overset{.}{m}\mspace{11mu} {Cp}} \right)\mspace{11mu} {air}}} & (6) \end{matrix}$

Where U is the global heat transfer coefficient and A is the area of heat transfer,

(T_(air)−T_(evap)) may be related to UA. Therefore, on measuring the temperature differences between the refrigerator chamber and the evaporator (T_(air)−T_(evap)), a relative efficiency under dry conditions is estimated which, following the mathematical relationships specified by the method, imply a UA_(dry).

Under frosted conditions, the same measurements generate a UA_(ice) value.

By means of the relationship UA_(ice)/UA_(dry) the aforementioned fc coefficient is determined; this is the relative level of frost and which enables the making of decisions regarding the management of the fan and the need to defrost.

As has been stated above herein, the calculation of the level of ice is performed at the commencement, precisely when a defrosting has been performed, and it is ensured that the evaporator is completely free from frost and under the stabilized thermal conditions. In accordance with the embodiment explained with regard to equations (1-6) this value, that is, the UA_(dry) value, is the reference (or value identified above as first value or reference value of the NTU rate). As the evaporator operates, the calculation of the level of frost, that is, the UA_(ice) value (identified above as the second NTU rate value) is performed periodically, and the value calculated is divided with regard to the reference (UA_(dry), frost-free). The division of both factors provides the fc value. From this explanation it may be deduced the value of fc=1 for the reference UA_(dry).

With regard to the frequency of calculation for the production of the UA_(ice) value, this being that of the repetitions of said calculation, for one embodiment this is typically of 4 hours (one calculation every 4 hours), although this is parametrizable (the user may select a value between 2 and 6 hours). As the fc drops, approaching the fs value (the limit value indicating the need for defrosting), the frequency drops linearly to ensure that the evaporator is not blocked by frost; for example passing from 4 hours between calculations to 3 hours, and finally to 2 hours when is very close to fs.

With regard to what is referred above as the dimensionless reference performance coefficient fs, this may be understood to be a coefficient whose value indicates a lower limit for the fc value, in such a way that if the fc value drops until it reaches said lower limit, it is determined that a defrost is required. Specifically, for the embodiment detailed with reference to equations (1-6), this fs value (always between 0 and 1) represents the maximum tolerated reduction relative to UA_(dry) (frost-free) of the UA_(ice) (with a certain level of frost). Once this is reached (or surpassed downwardly, that is when fc<=fs), a defrost is commenced. By default, its value is relatively high (for example, 0.6) to prevent any blocking in the first iterations of the controller. As defrosts are performed, the times necessary for the melting of the frost in the equipment are measured. The greater the amount of frost in the evaporator, the longer the defrosting time. The fs coefficient is updated until defrosts of the desired length are achieved, by means of a defrosting strategy coefficient. Thus, the coefficient will commence, for example, at fs=0.6 (which means that the minimum acceptable UA_(ice) value in comparison with the UA_(dry) value is 60%). If said defrost entails a shorter defrosting time than desired, fs will be updated to, for example, 0.5, and at the next defrost it will again be assessed whether the amount of frost is equal to that desired, by means of the measurement of the defrosting time employed; and so on until reaching a fs value stabilized at the maximum amount of frost which is acceptable to the user.

Preferably, the method contemplates the existence of a safety indicator which can halt the refrigeration system and activate the defrosting process, in the event that this might be the reason for a malfunction.

Additionally, and thanks to the capacity of predicting the time for defrosting on the basis of the temporal evolution of the fc coefficient, the method contemplates that the heating system for drainage of the evaporator should only be activated when necessary (prior to defrosting) while it is maintained inactive during the periods where defrosting is not in operation or is not foreseen in the short term, which increases the potential savings which this adaptive method confers to the refrigeration system.

The principal advantages and innovative characteristics provided by the method of the invention are:

-   -   The NTU rate to quantify the level of frost in the evaporator.     -   The strategy for the fans (mode of operation) depends on the         level of frost in the evaporator. There exist several modes of         operation, depending on the level of frost.     -   The defrosting process is activated depending on an NTU rate in         the evaporator, which reduces the number of defrosts to be         performed.     -   The relative level of frost (NTU rate) to activate the defrost         adapts to the duration of the defrosting process, which may also         be related to the time during which the refrigerated space is         out of range.     -   On the basis of the temporal evolution of the NTU rate, the         drainage heating system is activated only when necessary, thus         increasing the potential energy savings in the system.

In short, the method comprises the detection of the level of frost in the evaporator by means of a calculation method of the NTU rate, which enables the definition of a) the most appropriate time for defrosting, b) the energisation of the drainage resistances, and c) the adaptive management of the evaporator fan combining different modes of operation, comprising an ice-free mode where solely the refrigeration capacity of the coolant is employed, and different iced modes where the latent heat stored in the ice is employed to provide energy savings, depending on the level of frost in the evaporator where, for the calculation of the NTU rate it uses as a reference the evaporator when it is dry, at the commencement, and when the refrigeration system is in operation, it performs the calculation of the NTU rate with a specific, precise fan management mode, carried out with a non-constant, but variable frequency, which varies depending on the performance of the evaporator or on the level of ice therein, and its comparison with the aforementioned reference.

In a second aspect, the present invention relates to an adaptive control method for refrigeration systems which, being of the type which manages the fans in accordance with the level of frost in the evaporator, comprises the detection of the level of frost in the evaporator by means of a calculation method alternative to that proposed by the first aspect, or second calculation method, whose scope of protection is to be found defined in claim 8.

Said second method provides an indicator representing the facility to the variation of temperature (FVT) of the evaporator, where the value of said FVT indicator drops with the amount of frost, as the mass of frost increases (greater thermal inertia), and reduces the power of heat transfer to the air (ε or heat exchange efficiency, as seen in the preceding method). The facility to the variation of temperature of the evaporator is calculated according to:

${F\; V\; T} = \frac{{Te\_ end} - {Te\_ ini}}{{timestep} \cdot {\sum{{abs}\left( \left( {T_{evap} - T_{air}} \right) \right)}_{i}}}$

where (Te_end−Te_ini) is the difference between the temperatures of the evaporator at the end and at the commencement of an evaporator heating (when there is no ingress of coolant into the same, the evaporator, with a ventilation activated, heats up until it reaches practically the temperature of the refrigerator chamber), (T_(evap)−T_(air)) are the successive samples of thermal gradient between evaporator and chamber which occur during said heating (a process which takes a number of minutes) and which are measured with each timestep (time in seconds between samples), where said factor is used to correct deviations in measurement caused by possible variations in temperature within the chamber.

Similarly to the indicated for the first method, based on E-NTU, from the values of the facility to the variation of temperature FVT of the evaporator under frost-free (dry) conditions, and under conditions with a certain level of frost (ice), the relative level of ice may be obtained by means of the relationship FVT_(ice)/FVT_(dry), represented by the fc coefficient.

The first method, that is, that of the first aspect of the present invention, is used when the evaporator cools the air in the refrigerator chamber by means of the evaporation of the coolant therein. Said value is calculated for a particular moment (generally a few seconds subsequent to the ingress of coolant into the evaporator). Conversely, the second method, this being the second aspect of the invention, is applied when the air in the refrigerator chamber heats the evaporator, with no ingress of coolant, which occurs during a process which is a question of minutes, during which thermal leaps between the air in the refrigerator chamber and the evaporator are averaged.

In view of the above, it is stated that the here described adaptive control method for refrigeration systems represents an innovation of characteristics unknown until now, for the purpose for which is intended; reasons which, in view of its practical utility, endow it with sufficient grounds to obtain the privilege of exclusivity requested.

DESCRIPTION OF THE DRAWINGS

As a complement to the description made herein, and for a better understanding of the characteristics of the invention, a drawing is attached to the present specification as an integral part thereof, wherein, by way of illustration but not limitation, the following is portrayed:

FIG. 1 portrays a flow diagram of the adaptive control method for refrigeration systems which is the object of the present invention, wherein the stages comprised by the method can be observed.

PREFERRED EMBODIMENT OF THE INVENTION

In view of the described and unique FIG. 1, and in accordance with the numbering adopted, it can be seen how the adaptive control method for refrigeration systems of the present invention contemplates the following stages in the order shown:

-   -   A first stage (1) wherein the default value of the fs         coefficient is predetermined, as is the maximum defrosting time         (tmax), which comprises reasonable values for the defrosting of         an evaporator of a refrigeration chamber (between 45 and 5 min).         For example, a default value of tmax=18 minutes is assigned,         this being parametrizable. The fs coefficient is adjusted until         the defrosting time reaches the value of tmax, which is         adjustable (parametrizable);     -   A second stage (2) where the evaporator is defrosted;     -   A third stage (3) where a standard operating mode of the fan is         executed, during a pre-set time or a time typical of the normal         operation of the regulation (control) of the cooling generation         within the refrigeration chamber. Said time is necessary for the         stabilisation of temperatures during the start-up of the         refrigeration chamber. It is generally set at half an hour,         although it is parametrizable;     -   A fourth stage (4), where the measurement operation mode is         executed, during a pre-set time;     -   A fifth stage (5), where the calculation of said first value or         reference value of the NTU rate is performed with the evaporator         dry, with no frost; a calculation performed at the commencement         of the regulation of cooling, subsequent to a defrost and always         subsequent to the pre-set time. Thus, it is ensured that the         evaporator is frost-free (thanks to the defrost) but the chamber         is under the thermal conditions stabilized to its normal         application (thanks to the pre-set time);     -   A sixth stage (6), where an initial/post-defrosting ice-free         operating mode of the refrigeration system is executed, wherein         solely the refrigeration capacity of the coolant is used;     -   A seventh stage (7), where the calculation of one of the second         values of the NTU rate is carried out, and also the obtaining of         the values of the fc coefficient of the relative level of frost         from said second value and said first value;     -   An eighth stage (8), where the calculation of the value of said         fc coefficient is carried out, with three possible options for         the following stage:         -   A ninth stage (9), where if the evaporator is frost-free,             the recurrent ice-free mode is executed; that is, using             solely the refrigeration capacity of the coolant;             subsequently returning to stage (7) where, once again, the             calculation of one of the second values of the NTU rate is             carried out, to obtain a new value of the fc coefficient of             the relative level of frost;         -   A tenth stage (10), where if the evaporator has a little             frost, the appropriate iced operation mode is executed,             depending on the value of said fc coefficient; that is, one             of the different iced modes is selected, where the latent             heat stored in the ice of the frost is employed to provide             energy savings; subsequently returning to stage (7) where,             once again, the calculation of one of the second values of             the NTU rate is carried out, to obtain the new fc             coefficient of the level of frost;         -   An eleventh stage (11) of thawing the evaporator, should             this have excessive frost; and     -   A twelfth stage (12), the performance of which is subject to the         performance of the eleventh stage (11), in which the value of         the fs coefficient of the level of frost is assessed and, if         deemed necessary, its value is adapted/updated, subsequently         returning to stage (6) wherein the initial/post-defrosting         ice-free fan operating mode is executed once again.

It should be noted that in order to perform said operating stages, the adaptive control method contemplates the input into the system of the following parameters:

-   -   Temperature of the evaporator     -   Temperature of the refrigerated space     -   Real Time Clock     -   Compressor ON/OFF signal     -   Solenoid ON/OFF signal     -   Defrost ON/OFF signal     -   Maximum acceptable defrosting time     -   Initial defrost activation coefficient (fs)     -   Safety time without defrosting     -   Hysteresis related to the temperature setpoint of the         refrigerated space     -   Maximum out-of-setpoint acceptable time

The nature of the present invention having been sufficiently described, as well as the manner of putting it into practice, it is not considered necessary to make the explanation thereof more extensive for any expert skilled in the art to understand its scope and the advantages to be derived therefrom; it is therefore stated that within its essential nature it may be put into practice in other embodiments which may differ in detail from that indicated by way of an example, embodiments which will be equally covered by the protection which is sought, provided that its fundamental principle is not altered, changed or modified. 

1. An adaptive control method for refrigeration systems which, being of the type which manages the fans according to the level of frost in the evaporator, comprising the detection of the level of frost in the evaporator using an NTU rate calculation method, and the adaptive management of the evaporator fan combining different operating modes, comprising an ice-free mode, where solely the refrigeration capacity of the coolant is employed, and different iced modes where the latent heat stored in the ice is employed to provide energy savings, depending on the level of frost in the evaporator; and wherein the method comprises the performance of said detection of the level of frost by by obtaining a dimensionless coefficient fc of the relative level of frost in the evaporator and the monitoring of the temporal evolution of the same, where the method comprises the obtaining of said dimensionless coefficient fc of the relative level of frost in the evaporator: from the calculation of a first value or reference value of the NTU rate, performed when the evaporator is dry at the commencement, with no frost, and from the calculation of second values of the NTU rate, when the refrigeration system is in operation during one of said iced modes of fan management, performing said calculation repeatedly over time, with a non-constant frequency of repetitions which varies depending on the performance of the evaporator or on the level of ice in the same; where said dimensionless coefficient fc of the relative level of frost in the evaporator relates, in a comparative manner, the second values with the first value of the NTU rate.
 2. The method of claim 1, wherein fc=UA_(ice)/UA_(dry), where U is a global heat transfer coefficient and A is the area of heat transfer, and they are obtained from the calculation of the aforementioned first and second values of the NTU rate, according to the following expression: UA=NTU·({dot over (m)}Cp)air where {dot over (m)} is the mass flow of air crossing the fins of the evaporator and Cp is the specific heat of the air, and NTU is obtained from the following expression: ε=1−e ^(−NTU) where ε is the efficiency of the heat exchange and is defined as ε_(dry) to calculate the first value of the NTU rate and as ε_(ice) to calculate the second values of the NTU rate, which are in turn related according to the following expression: ε_(dry)·(T _(air) −T _(evap))^(dry)=ε_(ice)·(T _(air) −T _(evap))^(ice) where (T_(air)−T_(evap))^(dry) is the temperature difference between the air in the refrigeration chamber and the evaporator when there is no frost/ice in the latter, and (T_(air)−T_(evap))^(ice) is the temperature difference between the air in the refrigeration chamber and the evaporator when there is frost/ice in the latter, and where the method comprises the measurement of the values of said temperatures.
 3. The method of claim 1, wherein in order to decide on the mode of operation and as to whether a defrosting process is necessary in real time, the value of the fc coefficient is compared with regard to a dimensionless reference performance coefficient fs, indicating that a defrost is necessary, where said value of fs is adapted, subsequent to said comparison of the values of fc with fs, being updated in accordance with the time required for the performance of the defrost by implementing one of said iced operating modes, on the basis of said fc value compared, the first fs value being a default value.
 4. The method of claim 3, wherein the method contemplates the existence of a safety indicator to halt the refrigeration system and activate the defrosting process, in the event that this might be the reason for a malfunction.
 5. The method of claim 3, wherein thanks to the capacity of predicting the time for defrosting on the basis of the temporal evolution of the fc coefficient, the method contemplates that the heating system for drainage of the evaporator should only be activated prior to defrosting, while it is maintained inactive during the periods where defrosting is not in operation or is not foreseen in the short term.
 6. The method of claim 3, wherein the method comprises the following stages: a first stage where the default value of the fs coefficient is predetermined, as is the maximum defrosting time (tmax); a second stage where the evaporator is defrosted; a third stage where a standard operating mode of the fan is executed; a fourth stage, where a measurement operation mode is executed; a fifth stage, where the calculation of said first value or reference value of the NTU rate is performed with the evaporator dry, with no frost; a sixth stage, where an initial/post-defrosting ice-free operating mode of the refrigeration system is executed, wherein solely the refrigeration capacity of the coolant is used; a seventh stage, where the calculation of one of the second values of the NTU rate is carried out, and also obtaining the values of the fc coefficient of the relative level of frost from said second value and said first value; an eighth stage, where the calculation of the value of said fc coefficient is carried out, with three possible options for the following stage: a ninth stage, where the recurrent ice-free mode is executed; that is, using solely the refrigeration capacity of the coolant; subsequently returning to stage where, once again, the calculation of one of the second values of the NTU rate is carried out, to obtain a new value for the fc coefficient of the relative level of frost; a tenth stage, where the appropriate iced operation mode is executed, depending on the value of said fc coefficient; that is, one of the different iced modes is selected, where the latent heat stored in the ice of the frost is employed to provide energy savings; subsequently returning to stage where, once again, the calculation of one of the second values of the NTU rate is carried out, to obtain the new fc coefficient of the level of frost; an eleventh stage of thawing the evaporator; and a twelfth stage, the performance of which is subject to the performance of the eleventh stage, of the adaptation/updating of the value of the fs coefficient of the level of frost, subsequently returning to stage, wherein the initial/post-defrosting ice-free fan operating mode is executed once again.
 7. The method of claim 1, wherein the method comprises obtaining said dimensionless coefficient fc of the relative level of frost in the evaporator when the evaporator is cooling the air within the refrigeration chamber of the refrigeration system via evaporation of the coolant circulating within the same.
 8. An adaptive control method for refrigeration systems which, being of the type which manages the fans according to the level of frost in the evaporator, comprising the detection of the level of frost in the evaporator by a calculation method of a FVT indicator representing the facility to the variation of temperature of the evaporator, according to the following expression: ${F\; V\; T} = \frac{{Te\_ end} - {Te\_ ini}}{{timestep} \cdot {\sum{{abs}\left( \left( {T_{evap} - T_{air}} \right) \right)}_{i}}}$ where (Te_end−Te_ini) is the difference between the temperatures of the evaporator at the end and at the commencement, respectively, of an evaporator heating process, (T_(evap)−T_(air)) are the successive samples of the thermal gradient between the temperature of the evaporator T_(evap) and that of the refrigeration chamber of the refrigeration system T_(air) occurring during said heating process, measured with each timestep or time in seconds between thermal gradient samples i.
 9. The method of claim 8, wherein the method comprises the performance of said detection of the level of frost by obtaining a dimensionless coefficient fc of the relative level of frost in the evaporator and the monitoring of the temporal evolution of the same, where the method comprises obtaining said dimensionless coefficient fc of the relative level of frost in the evaporator the relationship FVT_(ice)/FVT_(dry), where FVT_(ice) includes the values of the FVT indicator obtained when there is frost in the evaporator, and FVT_(dry) the values of the same when there is no frost in the evaporator. 